Radar level gauges are in wide use for making non-contact measurements of the level of products such as process fluids, granular compounds and other materials. These devices utilize antennas to transmit electromagnetic waves toward the material being monitored and to receive electromagnetic echoes which are reflected at the surface of the material being monitored. However, in a practical situation more than one radar echo usually can be seen and the dynamic range for the variation of all possible echoes is quite large.
However, a problem experienced in this type of level gauges is that the signal strength from a surface echo reduces as a function of measured distance. Typically, the signal strength reduces by 50% if the distance is doubled. As a consequence, the dynamic range in the receiver part is not optimally used. One method known in the art to compensate for this loss of signal strength is to amplify the received signal with a magnitude which is increased as a function of distance, so called IF-gain. Further, many radar level gauge systems have to work under low current and voltages and should preferably use low cost components. Thus, many of the classical methods for increased dynamic range may not be employable.
In order to solve the above-related problem, it has been proposed to increase the signal strength of the received signals in dependence of the distance from which the echoes originates. For example, U.S. Pat. No. 6,031,421 discloses a pulsed system for radar level gauging using sensitivity time control (STC), where the amplification in the receiver is controlled to provide a exponential gain with increased distance. U.S. Pat. No. 6,107,957 discloses a FMWC (frequency modulated continuous wave) radar level gauge using a similar amplification gain-control in order to provide an amplification inversely proportional to the distance from which the echoes originate.
However, a problem with these known systems is that they are relatively insensitive and static, and unable to adapt to the specific conditions related to the tank in which they are to be used, Specifically, the known systems are conventionally dimensioned for a maximal measuring distance, e.g. 30 meters. However, in practical use the tanks are of varying height, whereby non-optimal amplification is provided.
Still further, most known systems have problems related to the noise level, due to the increased noise level generated by the amplification. This is particularly disadvantageous in measuring systems subjected to high noise levels.
It is therefore a need for a more effective amplification control for use in level gauging.